Birds have balance organs in their butts. Why is no-one talking about this?!

January 23, 2019

Birds have little blobs of tissue sticking out on either side of the spinal cord in the lumbosacral region (solid black arrow in the image above). These are the accessory lobes of Lachi, and they are made up of mechanosensory neurons and glycogen-rich glial cells (but they are not part of the glycogen body, that’s a different thing that lies elsewhere — see this post).

These accessory lobes have been known since at least 1889, when they were first described by Lachi. But the function was mysterious until recently.

Starting in the late 1990s, German anatomist and physiologist Reinhold Necker investigated the development, morphology, and function of the lumbosacral canals of birds. These are not pneumatic spaces, they’re fluid-filled tubes that arch above (dorsal to) the spinal cord in the lumbosacral regions of birds. In a sacral neural canal endocast they look like sets of ears, or perhaps caterpillar legs (below image in the above slide).

Here’s the same slide with the top image labeled, by me.

In our own bodies, the meningeal sac that surrounds the spinal cord is topologically simple, basically a single long bag like a sock with the spinal cord running through the middle. In the lumbosacral regions of birds, the meningeal sac is more like a basket in cross-section, with dorsally-arching loops — the lumbosacral canals — forming the basket handles (lower image in the above slide). Evidently cerebrospinal fluid can slosh through these meningeal loops and push on the accessory lobes of Lachi, whose mechanosensory neurons pick up the displacement. This is essentially the same system that we (and all other vertebrates) have in the semicircular canals in our inner ears, which give us our sense of equilibrium.

Evidence that the lumbosacral canals function as organs of equilibrium comes not only from anatomy but also from the behavior of experimentally-modified birds. If the lumbosacral canals are surgically severed, creating the ‘lesion’ mentioned in the above figure, the affected birds have a much harder time controlling themselves. They can do okay if they are allowed to see, as shown on the left side of the above figure, but if they are blindfolded, they don’t know how to orient themselves and flop around clumsily. Meanwhile, blindfolded birds with their lumbosacral canals intact can balance just fine.

All of this is documented in a series of papers by Necker and colleagues — particularly useful are Necker (1999, 2002, 2005, 2006) and Necker et al. (2000). Necker (2006) seems to be the summation of all of this research. It’s very well-documented, well-reasoned, compelling stuff, and it’s been in the literature for over a decade.

So why is no-one talking about this? When I discovered Necker’s work last spring, I was stunned. This is HUGE. In general, the central nervous systems of vertebrates are pretty conserved, and animals don’t just go around evolving new basic sensory systems willy-nilly. Minimally I would expect congressional hearings about this, broadcast live on C-SPAN, but ideally there would be a talk show and a movie franchise.

I was equally blown away by the fact that I’d never heard about this from inside the world of science and sci-comm. Necker’s discovery seemed to have been almost entirely overlooked in the broader comparative anatomy community. I searched for weaknesses in the evidence or reasoning, and I also searched for people debunking the idea that birds have balance organs in their butts, and in both cases I came up empty-handed (if you know of counter-evidence, please let me know!). It’s relevant to paleontology, too: because the lumbosacral canals occupy transverse recesses in the roof of the sacral neural canal, they should be discoverable in fossil taxa. I’ve never heard of them being identified in a non-avian dinosaur, but then, I’ve never heard of anyone looking. You can also see the lumbosacral canals for yourself, or at least the spaces they occupy, for about three bucks, as I will show in an upcoming post.

Incidentally, I’m pretty sure this system underlies the axiomatic ability of birds to run around with their heads cut off. I grew up on a farm and raised and slaughtered chickens, so I’ve observed this firsthand. A decapitated chicken can get up on its hind legs and run around. It won’t go very far or in a straight line, hence the jokey expression, but it can actually run on flat ground. It hadn’t occurred to me until recently how weird that is. All vertebrates have central pattern generators in their spinal cords that can produce the basic locomotor movements of the trunk and limbs, but if you decapitate most vertebrates the body will just lie there and twitch. The limbs may even make rudimentary running motions, but the decapitated body can’t stand up and successfully walk or run. Central pattern generators aren’t enough, to run you need an organ of balance. A decapitated bird can successfully stand and run around because it still has a balance organ, in its lumbosacral spinal cord.

You may recognize some of the slides that illustrate this post from the Wedel et al. (2018) slide deck on the Snowmass Haplocanthosaurus for the 1st Palaeontological Virtual Congress. Those were stolen in turn from a much longer talk I’ve given on weird nervous system anatomy in dinosaurs, which I am using piecemeal as blog fuel. Stay tuned!

So, birds have balance organs in their butts. We should be talking about this. The comment thread is open.

References

  • Lachi, P. 1889. Alcune particolarita anatomiche del rigonfiamento sacrale nel midollo degli uccelli. Lobi accessori. Att Soc Tosc Sci Nat 10:268–295.
  • Necker, R. 1999. Specializations in the lumbosacral spinal cord of birds: morphological and behavioural evidence for a sense of equilibrium. European Journal of Morphology 37:211–214.
  • Necker, R. 2002. Mechanosensitivity of spinal accessory lobe neurons in the pigeon. Neuroscience Letters 320:53–56.
  • Necker, R. 2005. The structure and development of avian lumbosacral specializations of the vertebral canal and the spinal cord with special reference to a possible function as a sense organ of equilibrium. Anatomy and Embryology 210:59–74.
  • Necker, R. 2006. Specializations in the lumbosacral vertebral canal and spinal cord of birds: evidence of a function as a sense organ which is involved in the control of walking. Journal of Comparative Physiology A, 192(5):439-448.
  • Necker, R, Janßen A, Beissenhirtz, T. 2000. Behavioral evidence of the role of lumbosacral anatomical specializations in pigeons in maintaining balance during terrestrial locomotion. Journal of Comparative Physiology A 186:409–412.
  • Wedel, M.J., Atterholt, J., Macalino, J., Nalley, T., Wisser, G., and Yasmer, J. 2018. Reconstructing an unusual specimen of Haplocanthosaurus using a blend of physical and digital techniques. Abstract book, 1st Palaeontological Virtual Congress, http://palaeovc.uv.es/, p. 158 /  PeerJ Preprints 6:e27431v1
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20 Responses to “Birds have balance organs in their butts. Why is no-one talking about this?!”

  1. Brad Lichtenstein Says:

    Uh, since you asked…
    Uh…
    Since it appears from the hood studies that they don’t have the semicircular canals in their ears, this explains why pigeons can do the pigeon without confusing their balance?

    More broadly, I’m told vertebrates adapted to the ocean don’t have functional semicircular canals, so they can maneuver in 3D without ¿getting dizzy or otherwise directionally confused?, so maybe something similar is useful for flight? But something similar is obviously needed for walking especially in the dark, or maybe maintaining level flight (picturing especially the raptors and owls turning their heads in search of prey while maintaining level flight).

    Uh… Good for them?

  2. Brad Lichtenstein Says:

    Now I’m wondering if one of my college professors, who did the pigeon, was really more than half bird?

  3. Andrea Cau Says:

    Ok, now let’s check all dinosaur sacrals to find the same structure.

  4. Matt Wedel Says:

    Brad wrote:

    Since it appears from the hood studies that they don’t have the semicircular canals in their ears, this explains why pigeons can do the pigeon without confusing their balance?

    First, all of those pigeons do have functional semicircular canals in their ears, but without visual input they have a hard time integrating balance information (as do we). Second, I think pigeons “do the pigeon” precisely to maintain balance. The head-bobbing is in sync with the hindlimb step cycle, and I suspect that it’s to keep the center of mass over the feet even though the hip joints are so far back on the body.

    Sorry if I blew your joke with serious answers!

    More broadly, I’m told vertebrates adapted to the ocean don’t have functional semicircular canals, so they can maneuver in 3D without ¿getting dizzy or otherwise directionally confused?

    Pretty sure that’s incorrect. First, semicircular canals first evolved in an aquatic context, and second, there have been extensive studies of cetacean semicircular canals and they are small but definitely functional.

    The idea that a second balance organ in the sacrum might be useful for flying species is definitely intriguing. A counterpoint is that the lumbosacral canal system is larger and develops earlier in predominantly terrestrial birds. I suspect it correlates with amount of time spent walking, and might be another consequence of having the hip joint all the way at the back of the body. Even with predominantly knee-driven walking and running, birds may need balance organs at both ends to keep everything moving properly.

    Andrea wrote:

    Ok, now let’s check all dinosaur sacrals to find the same structure.

    Quite! It should be easy to test if the development of this system is linked to knee-driven rather than hip-driven locomotion, or if it correlates with something else entirely.

  5. llewelly Says:

    Does this mean birds fly by the seat of their pants??

  6. LeeB Says:

    It would be interesting to know if Pterosaurs have something similar.
    LeeB.

  7. Anonymous Says:

    Even lampreys and hagfish have semicircular canals (but reduced numbers of them compared to gnathostomes).

  8. Mike Taylor Says:

    The head-bobbing is in sync with the hindlimb step cycle, and I suspect that it’s to keep the center of mass over the feet even though the hip joints are so far back on the body.

    I doubt that’s the cause. My flawless intuition tells me that head mass is not very significant in this regard. Synchronisation of head-bobbing with step cycle is more likely to do with maintaining a stable visual field. (But note that I do not know what I’m talking about. Likely, someone has studied this and knows the answer.)

  9. Marja Erwin Says:

    If it’s a bipedal adaptation, Poposaurs may also be relevant.

  10. Dann Says:

    I wonder if this adaptation helps birds to perch in trees at night? Diurnal volant birds that spend the night perching in a tree need to be able to keep their balance in the dark. A balance organ independent of the brain would also be useful for keeping balance while the brain is sleeping. They have to do all that on branches that may be swaying in the breeze as well.

    A tight perching grip would surely help, but without a way of balancing while asleep they might risk waking the next morning hanging upside-down (like a galah playing on a power line).

  11. Matt Wedel Says:

    To my immense irritation, Mike’s intuition seems to be correct. Or at least not yet falsified. Link.

  12. Mike Taylor Says:

    *fist-pump*

    That treadmill experiment looks like a slam-dunk.

  13. David Marjanović Says:

    Fascinating. I had no idea.

  14. Don Cox Says:

    Surely pigeons bob their heads to get stereo vision ? Try moving your head from side to side to get the same effect.


  15. This is being studied by Dr. Monica Daley et al (in birds) with anatomy, models, experiments, robots etc:
    https://blog.lifescitrc.org/ugresearch/2018/09/28/do-birds-have-a-butt-brain/
    https://www.frontiersin.org/articles/10.3389/frobt.2018.00038/full
    Not only Necker’s work has been somewhat forgotten, but the related paleontology work by Giffin (now Bucholz) is neglected.

  16. Mike Taylor Says:

    Sorry, John, your comment should not have been held for moderation: I guess WordPress erred on the side of caution because it included two URLs. So you posted it again: I’ve modded one through, and trashed the slightly less informative of the two near-duplicates.


  17. Thanks Mike! Chrome decided to crash on my 1st comment, oddly enough.

  18. Matt Wedel Says:

    Thanks for the heads up about Monica Daley’s work, John. I’m glad to know that someone is on this! Way too cool of a discovery to not get significant further investigation.

  19. Leo Sham Says:

    It seems, though a wild guess it is, no one has yet suggested that it was evolved in response to the loss of the ancestral tail and subsequent shift of the center of mass. Surely, the posture adapted (femoral orientation) but the accompanying change to the joint proprioception signal input might have been awkward to those early birds. More neural coordination was thus needed. If this were the case then we may not expect to find the lumbosacral enlargement of the assessory lobes outside of e.g. Pygostylia,

  20. Matt Wedel Says:

    Yeah, I think that’s very likely. I may not have conveyed it very clearly, but that’s what I was alluding to in the fourth comment when I wrote that it “might be another consequence of having the hip joint all the way at the back of the body. Even with predominantly knee-driven walking and running, birds may need balance organs at both ends to keep everything moving properly.”

    You are right about the lumbosacral canals probably not being present outside of Pygostylia if that’s the case. Should be straighforward to test if there are uncrushed sacra on either side of the divide available for CT scanning.


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